Japanese Navy 13 mm machinegun cartridge?


I have another cartridge case that I collected at a WWII site on Pohnpei (formerly Ponape) Island in Micronesia during my archaeological survey last year.

It is a broken cartridge case which measures 20.5 mm in diameter of the rim. The cartridge bore the headstamp: “U” at 12 o’clock, “2” at 4 o’clock, and “4” at 8 o’clock. (I like to post a photo, but have a problem again…)

My friend identified wrote me that it was for a Japanese Navy 13 mm machinegun. According to an US intelligence report (Bureau of Ordnance, 1946. Japanese Explosive Ordnance. Ordnance Pamphlet 1667. Washington, D.C.: Bureau of Ordnance, Navy Department.), it was probably used for “Type 93 AAMG” or “Type 3 fixed/flexible ACMG”.

Please let me know any information on this cartridge case (e.g., headstamp, gun type). Thank you for your information.

Takuya Nagaoka
PhD Student
Department of Anthropology
University of Auckland
New Zealand
Email: takuya.nagaoka@gmail.com
Website: arts.auckland.ac.nz/staff/index.cfm?P=11002

If you want to email me the photo I’ll post the picture on the forum.

I would have to say it’s an American .50 Cal case they have a rim dia of approx 20.5 mm

the head stamp “U 42” is more then likely a US maker (I can’t remember who U is at this stage)

Here is the photo of the round:

Kevin is correct, .50 BMG (12.7x99) caliber US, heavy machine gun. Utah Ordnance Plant, 1942 production.

Keith Pagel
FCSA / VHP Magazine

For information, the Japanese Naval 13mm Type 93 (Hotchkiss M1930) AA gun and the 13mm Type 3 Naval aircraft gun (a slightly modified .50 Browning) were both chambered for the 13.2x99 Hotchkiss round, which was identical to the .50 BMG (12.7x99) except for the fractionally larger calibre.

In this case, it’s the headstamp which is definitive.

Thank you very much for your correction! I am very glad to know that I have correct information now.

The case was found at a concrete foundation of a Japanese military building. The island was heavily bombed by US airplanes in 1944. So, is it reasonable to assume that the machinegun was set up on an US aircraft and the case was dropped from the airplane during the air raid?


Considering the Browning Machine Gun, Aircraft Cal. .50, M2 was mounted in nearly every US aircraft in WW2, I’m betting this is the remnant of an aerial dogfight or a strafing run. Great find.